If you look at the poll details here [PDF], you see that of the 67% who believe that there is solid evidence that the average temperature on Earth has been getting warmer over the past four decades, 42% say that the cause is human activity. So 42% of 67% = 28% of Americans think that the Earth is getting warmer because of human activity.

I'd like to see some follow-up questions for that 28%, along the lines of "Do you think that this warming is very dangerous?", and "Would you personally be willing to cough up $100 cash every month for federal government efforts to stop this warming?".